The present invention relates to electrophotographic printing and more specifically to a method for maintaining a preselected developed mass per unit area (DMA) of black solid area in a xerographic printer by adjusting the development field of the system during run-time operation. More specifically, the invention relates to a method where the DMA of a black solid area control patch, which cannot be sensed directly using a toner density sensor, is estimated to allow for improved accuracy in the regulation of the DMA by adjusting the development field.
In the well-known process of electrophotographic printing, a charge retentive surface, typically known as a photoreceptor, is electrostatically charged, and then exposed to a light pattern of an original image to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on the photoreceptor form an electrostatic charge pattern, known as a latent image, conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder, known as "toner". Toner is held on the image areas by the electrostatic charge on the photoreceptor surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate or support member such as paper, and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is useful for light lens copying from an original document or for printing electronically generated or stored originals such as with a raster output scanner (ROS), where a charged surface may be imagewise discharged in a variety of ways.
In such electrophotographic printing, the step of conveying toner to the latent image on the photoreceptor is known as "development." The object of effective development of a latent image on the photoreceptor is to convey toner particles to the latent image in a controlled manner so that the toner particles effectively adhere electrostatically to the charged areas on the latent image. A commonly used technique for development is the use of a two-component developer material, which comprises, in addition to the toner particles which are intended to adhere to the photoreceptor, a quantity of magnetic carrier beads. The toner particles adhere tribolelectrically to the relatively large carrier beads, which are typically made of steel. When the developer material is placed in a magnetic field, the carrier beads with toner particles thereon form what is known as a magnetic brush, wherein the carrier beads form relatively long chains which resemble the fibers of a brush. This magnetic brush is typically created by means of a "developer roll." The developer roll is usually in the form of a cylindrical sleeve rotating around a fixed assembly of permanent magnets. The carrier beads form chains extending from the surface of the developer roll, and the toner particles are electrostatically attracted to the chains of carrier beads. When the magnetic brush is introduced into a development zone adjacent the electrostatic latent image on the photoreceptor, the electrostatic charge on the photoreceptor will cause the toner particles to be pulled off the carrier beads and onto the photoreceptor.
An important variation to the general principle of development is the concept of "scavengeless" development. In a scavengeless development system, toner is detached from a donor roll by applying an AC electric field to self-spaced electrode structures, commonly in the form of wires positioned in the nip between a donor roll and photoreceptor. This forms a toner powder cloud adjacent thereto. Because there is no physical contact between the development apparatus and the photoreceptor, scavengeless development is useful for devices in which different types of toner are supplied onto the same photoreceptor such as in "tri-level", "recharge, expose and develop", "highlight", or "image on image" color xerography.
Typically, area development control is established by creating toner control patches of single desired density. Control patches are created using an alternate light source, such as a patch generator, to properly discharge the photoreceptor to the proper development field. The actual developed mass per unit area (DMA) of the toner on the control patches is then optically measured to determine the effectiveness of the printing process in placing the toner on the print sheet. Typically, a reflection infra-red densitometer is used for determining the density of the toner on a control patch. Both solid area and halftoned control patches of varying densities, including a black solid area control patch, can be used to assure color quality control. Solid patches are represented on a Solid Area Developability Curve and halftoned patches are represented on a Tone Reproduction Curve (TRC).
Direct measurement of a black solid area control patch with a reflection infra-red densitometer (IRD) is problematic in many xerographic copiers and printers. The reflection IRD is limited in the range of DMA that it can sense on a control patch. The reflection IRD cannot sense the black full developed mass of toner on a control patch when the developed mass is beyond a given limit. Typically this limit is below the black mass necessary to achieve the desired image darkness. In response to this limitation, the DMA of black solid area control patches is often estimated using a single lower density test patch created by the patch generator.
With reference to FIG. 1, a plot of DMA versus development voltage V.sub.dev is provided to illustrate estimation techniques of the prior art. Typically, xerographic copiers and printers are equipped with a nominal developability curve, as illustrated in FIG. 1. The nominal developability curve provides the toner density target for the system at a given development voltage. Due to environmental conditions, such as changes in humidity, and/or consumption of toner, the developability curve often changes slope from the nominal curve. For example, regular consumption of toner at a rate greater than replenishment may result in a lower toner concentration (TC), and therefore, a higher triboelectrification of the toner. This condition results in a visibly "lighter" solid black area in printed images, i.e. a lower DMA at a given development voltage. Such a condition is represented by a developability curve with a slope which is less than the slope of the nominal developability curve. Because the reflection IRD cannot sense such a change in the DMA of the solid black control patch, the DMA of the control patch must be estimated using extrapolation.
The conventional method of adjusting toner density consists of adding or removing toner from the development housing, i.e. adjusting the TC. When the TC is adjusted and the development field is held constant, the developability curve is fixed at a development onset V.sub.DO, as shown in FIG. 1. Therefore, adjusting the toner concentration simply rotates the developability curve by changing its slope with a fixed development onset of V.sub.DO. Because the change in DMA of the solid black area control patch cannot be sensed directly with an IRD, the DMA at the reduced TC must be estimated. The estimation technique of the prior art consists of generating a test patch using the patch generator at a DMA that is within the sensing range of the IRD. The DMA of the test patch is sensed using the IRD and the development voltage V.sub.pgen of the test patch is sensed using an electrostatic voltmeter (ESV). The sensed DMA reading of the test patch is then compared to the preselected DMA reading of the nominal curve at the development voltage V.sub.pgen. From this data, the DMA of a solid black area control patch along the reduced TC developability curve is estimated. Based on the estimated DMA of the black solid area control patch, TC is adjusted to return the black solid area of the system to its preselected toner density.
Adjusting TC to correct the toner density of black solid area necessarily has a much slower response than adjusting other parameters, such as development field. Such a slow response especially adversely affects the maintenance of color quality control. Adjusting the development field in order to maintain toner density provides a much faster response. Accordingly, there is a need for a black solid area estimation technique which reliably compensates for system changes.
It is an object of the present invention to provide a new and improved method for maintaining a preselected DMA of solid black area by adjusting development field and estimating the DMA of a solid black control patch which overcomes the above-referenced problems and others.